The present invention relates to a second harmonic generating method and apparatus for improving an unstable phenomenon of light output occurring by a non-linear optical device.
In a magneto-optical recording method for recording/reproducing data by using laser beam, an 830 nm-infrared semiconductor laser is being used. However, there is a difficulty in pursuing higher recording densities. In focusing, the 830 nm-semiconductor laser has a focal diameter over 830 nm, which hinders high-density recording. If the laser source has a shorter wavelength than 830 nm, its focal diameter can be reduced accordingly.
Such a short-wave laser source, comprising a helium-neon laser and an argon laser, is difficult to deal with because it is too large for commercial-use data recording and playback functions and consumes an excessive amount of power.
A second harmonic generator pumps a Nd:YAG crystal with an 809 nm-semiconductor laser which is small in volume and is thus easy to handle, and thereby forms a 1064 nm-laser source. The second harmonic generator uses a non-linear device made of KTP (specifically, KTiOPO.sub.4 or potassium titanyl phosphate) or KNbO.sub.3 so as to obtain a second harmonic (i.e., a 532 nm-laser which is half the wavelength of the 1064 nm-laser) with respect to the fundamental.
Since the second harmonic generator uses a nonlinear optical device being very sensitive to temperature fluctuations, its light output is unstable. A uniform laser output can be obtained only when the temperature is precisely controlled.
Referring to FIG. 1 showing a conventional apparatus, an 809 nm laser emitted from a laser diode 100 passes through a first mirror 102 and a Nd:YAG crystal 104 located inside a resonator between two mirrors 102 and 116, resulting in a laser resonance of a 1064 nm fundamental wave. Here, a Brewster plate 106 and a nonlinear optical device 110 are located between Nd:YAG crystal 104 and second mirror 116. A 532 nm second harmonic, which is half the wavelength of the fundamental, is generated from nonlinear optical device 110.
1064 nm- and 532 nm-light beams are mixed in the light beam proceeding to second mirror 116. However, second mirror 116 reflects the 1064 nm light and outputs only the 532 nm light because it has a high reflection rate with respect to the fundamental. Since part of the 1064 nm light remains in the light passing through the second mirror, a blocking plate 118 is located behind second mirror 116 so as to transmit the 532 nm light and interrupt the 1064 nm light. In order to use part of the 532 nm light output via blocking plate 118 in output light stabilization, a beam splitter 120 is provided behind blocking plate 118 so that part of the 532 nm light is input to a photodetector 122.
When the 532 nm laser beam (second harmonic) reaches photodetector 122, the beam is photoelectrically converted so as to become a current in proportion to the amount of input light. The current is input to a second harmonic light output controller 124.
A control signal output from second harmonic light output controller 124 is input to a thermo-electric cooler 112 so as to control the temperature of nonlinear optical device 110.
The configuration and operation of second harmonic light output controller 124 will be described below with reference to FIG. 2.
Second harmonic light output controller 124 is made up of an amplifier 200, a second harmonic light output setting portion 202, a comparator 204, an integrator 206, and thermo-electric cooler driving portion 208.
Here, since the back surface of nonlinear optical device 110 becomes very hot, a heat sink 114 is provided to spontaneously emit the heat of a high-temperature portion, thereby enhancing the efficiency of the thermo electric cooler and effectively controlling the temperature of a low-temperature portion.
The current from photodetector 122 for generating the current corresponding to the light output from nonlinear optical device 110 is input to amplifier 200 for the purpose of light output stabilization and is amplified therein to output a voltage, which is in turn input to comparator 204 for the comparison with a set voltage of second harmonic light output setting portion 202.
The comparison value of comparator 204, which is a light output offset signal to the set value of a light output, is integrated by integrator 206 so that thermo-electric cooler driving portion 208 drives thermo-electric cooler 112 and therefore controls the temperature of nonlinear optical device 110.
As a result, the control mechanism of the conventional second harmonic generator is designed to control the temperature of nonlinear optical device 110 with the harmonic light output therefrom.
FIG. 3 is a circuit diagram of the light output controller according to the conventional technology.
Referring to FIG. 3, light output comparator 608 is comprised of a photodiode PD1, resistors R1-R5, R7 and R8, a variable resistor VR1, and operational amplifiers OP1 and OP2.
Here, photodiode PD1 applies a voltage proportional to the light output amount from nonlinear optical device 110 to the inverting port of operational amplifier OP1. Operational amplifier OP1 amplifies the input signal and outputs the amplified signal to the inverting port of operational amplifier OP2. Operational amplifier OP2 compares the voltage signal output from operational amplifier OP1 and the voltage formed by resistors R7 and R8 and variable resistor VR1, thereby outputting a voltage corresponding to the difference between the two signals. To perform temperature control of nonlinear optical device 110, thermo-electric cooler driving portion 208 receives the integration signal from integrator 206 via the base of transistor TR1 and provides a driving current to thermo-electric cooler 112 in proportion to the integration signal.
FIG. 4 shows a graph indicating the relationship between temperature and light output of the nonlinear optical device, with several peaks of light output. In FIG. 4, for a stable output, the light output offset is negatively fed back until the light output set value set by second harmonic light output setting portion 202 is equal to an actually obtained light output.
The light output offset is applied to thermo-electric cooler 112 so as to lower the temperature of nonlinear optical device 110. In FIG. 4, at temperature T.sub.1 which is the initial temperature of the nonlinear optical device, a negative feedback is performed to lower the temperature of nonlinear optical device 110 until the temperature thereof becomes temperature T.sub.2 at which the second harmonic light output reaches the light output set value. Then, at the first peak, the light output increases and the temperature stops changing at the light output set value.
However, the first peak has a smaller maximum margin of light output than the second peak does. In other words, since it is hard for the light output to be uniformly maintained according to the nonlinear optical device and second harmonic generator, the second peak has a higher stability of light output than the first peak does.
As shown in FIG. 5A showing the variation of light output over time in the conventional harmonic generator, the temperature of the nonlinear optical device moves from T.sub.2 to T.sub.3. Here, there is created a light output unstable section B in which the light output is smaller than the light output set value. When the temperature continues to move toward T.sub.3 via the light output unstable section and the light output becomes equal to the light output set value, the temperature of the nonlinear optical device stops changing.
However, as shown in FIG. 5B, for the light output unstable section, the variation amount of temperature over time is the highest in the nonlinear optical device. In other words, since the temperature control is performed via section A, unstable section B and stable section C, the light output profile contains unstable sections.